Complete knee prosthesis

ABSTRACT

A prosthesis whose condyles ( 3 ) have a spiral form, viewed in the saggital plane and the femoral element ( 1 ) has a medial lobe ( 6 ) with a curved surface. The medial plateau ( 2 ) has glenoid cavities ( 2   b ) for the condyles ( 3 ) and a medial prominence ( 5 ) which forms a posterior support surface ( 5   a ) against which the lobe ( 6 ) is supported in the course of movement of the joint. The glenoid cavities ( 2   b ) on the medial plateau ( 2 ) and the condyles ( 3 ) are congruent in extension and in the first part of the flexion movement of the joint. The posterior sections of the condyles ( 3 ) and the medial lobe ( 6 ) inscribe circles with the same centres and the medial lobe ( 6 ) contacts against the posterior support surface ( 5   a ) at the end of the first part of movement and then rests against the posterior support surface ( 5   a ) to perform a “roll-back”.

The present invention relates to complete knee prosthesis. Suchprosthesis comprises, as it is well known, a femoral element reproducingthe femoral condyles, a tibial seat forming an upper support plate and amedial plateau made of a material promoting a slipping movement, notablyof high density polyethylene. This medial plateau has a lower faceintended for resting against the upper support plate of the tibial seatand glenoid cavities receiving the condyles of the femoral element.

In a complete knee prosthesis, it is advisable that the prostheticcondyles reproduce globally the form of the natural condyles of the kneejoint, to obtain good reproduction of the movement of this joint and ofan adequate ligamental tension. These prosthetic condyles must thereforehave, as seen in the sagittal plane, a spiral form correspondingsubstantially to that of the natural condyles, i.e. with gradualreduction of their radii towards the posterior sectors.

There results from these reduced radii that the contact surface betweenthe condyles and the medial plateau varies with the flexion movement orthe extension of the joint.

Certain prostheses promote the congruence in extension, i.e. between 0and 40° to 60° approx. in flexion. In such a case, the contact of theprosthetic condyles with the medial plateau, beyond these angles offlexion, becomes linear or even punctual, and non surface-related, whichenables the femur to move forward with respect to the tibia, accordingto a so-called “posterior drawer” movement. This movement is contrary tothe physiologic recall movement of the femur with respect to the tibiaduring the so-called “roll-back” flexion.

It is advisable to reproduce this “roll-back” movement on a kneeprosthesis. Indeed, this movement enables to increase the extensionforce of the quadriceps, while increasing the lever arm thereof.Moreover, this movement enables to delay the moment when the femur abutsagainst the medial plateau at the end of the flexion and to reduce thecamming effect of the soft portions in this very position, whichincreases the amplitude of movement of the joint and limits the stresseson the bony anchoring interfaces of the femoral element and of thetibial seat.

To solve the problem of so-called “posterior drawer” movement, it isknown to adjoin to a knee prosthesis a “postero-stabilisation” system,which consists of a stop between a bar or a lobe provided on the femoralelement, between both condyles, and a medial prominence of the medialplateau. This system limits the forward movement of the femur withrespect to the tibia, but the contact surface between the femur and themedial element is necessarily linear or punctual in flexion. Thislimited contact surface is a source of wear for the medial plateau andis therefore detrimental to the perennity thereof.

Certain prostheses enable surface congruence in extension and flexion:the posterior sagittal condylian radius inscribes a circle, and themedial plateau has glenoid surfaces with corresponding circular profile.In such a case, the surface contact is maintained throughout theflexion, but, taking into account this congruence, the translationmovements of the femur with respect to the medial plateau are limited orimpossible.

Certain prostheses enable antero-posterior mobility between the medialplateau and the tibial element. In such a case, the movement of thefemur with respect to the tibia results not from the form of the contactsurfaces but from the action of the muscles and of the ligaments. The“roll-back” movement aforementioned being essentially controlled by theposterior cross ligament, these prostheses impose the conservation ofthis posterior cross ligament, which is not always possible, requestedor advisable.

The purpose of this present invention is to remedy the shortcomingsaforementioned of the extent complete knee prostheses, while supplying a“postero-stabilisation” prosthesis enabling to maintain high congruencebetween the femoral element and the medial plateau over the wholemovement of the joint, while enabling perfectly controlled “roll-back”movement, and without imposing the conservation of the posterior crossligament.

The prosthesis affected comprises, in a manner known in itself, afemoral element, a tibial seat and a medial plateau as aforementioned,the femoral condyles having, seen in the sagittal plane, a spiral formcorresponding substantially to that of the natural condyles, i.e. withgradual reduction of their radii towards posterior sectors, and themedial plateau having a medial prominence which forms a posteriorsupport surface, while the femoral element has a medial lobe with acurved surface, capable of resting against this posterior supportsurface during the movement of the joint.

According to the invention,

-   -   the glenoid cavities of the medial plateau and the condyles of        the femoral element are formed to be congruent when the        prosthesis is in extension and on a first part of the flexion        movement of the joint extending between this extension position        and the angle of flexion from which the congruence of the        condyles with the glenoid surfaces is lost due to the spiral        form of the condyles;    -   the posterior sectors of the condyles and of the medial lobe        inscribe circles of same centres;    -   the medial lobe is formed so as not to contact said posterior        support surface during said first part of movement, but to        contact said posterior support surface when said flexion angle        is reached and then to carry the joint against this posterior        support surface over the remainder of the flexion travel, while        performing a “roll-back”, i.e. a rolling movement of the femoral        condyles with respect to the tibial seat in the posterior        direction, and    -   said posterior support surface has a curved shape corresponding        to that of said medial lobe, so that this lobe rests against        this posterior support surface over a wide surface throughout        the second part of movement, consecutive to said first part of        movement.

The prosthesis according to the invention thus comprises femoralcondyles of spiral form having a congruence with the medial plateau onsaid first part of flexion movement. This form and this congruenceenable good reproduction of the movement of the natural joint and goodcontrol of the movement of the femur with respect to the tibia andconversely.

At said angle of flexion, the medial lobe contacts said posteriorsupport surface, according to a soft and gradual movement resulting fromcurved forms complementary to this lobe and to this surface. This softand gradual contact avoids any shock liable to stress the bonyanchorings.

From this flexion angle, the resting of the lobe against said posteriorsupport surface compensates for the loss of congruence of the femoralcondyles with the medial plateau, which enables to keep the control ofthe movement of the femoral element with respect to the tibial seat andconversely; the fact that the posterior sectors of the condyles and ofthe medial lobe inscribe circles of same centres enables to perform the“roll-back” movement, which enables not only to reproduce the anatomicmovement, but also to increase the amplitude of the flexion movement ofthe joint and not to generate detrimental stresses on the bonyanchorings at the end of this flexion movement.

The medial plateau is advantageously mobile with respect to the tibialseat in order to limit the stresses exerted on this plateau on the bonyanchorings. Notably, this plateau may be mounted to pivot on the tibialseat by means of a stud included in the medial plateau or the tibialseat, engaged in a corresponding cavity provided respectively in theseat or the medial plateau.

An embodiment of the prosthesis according to the invention is describedbelow with reference to the appended schematic drawing, wherein:

FIG. 1 is a sagittal view thereof, as a medial antero-posteriorsectional view;

FIGS. 2, 3, 5, 7 and 9 are sagittal views, as medial antero-posteriorsectional views, of the femoral element and of the medial plateau ofthis prosthesis, respectively in extension position of the prosthesisand according to 30°, 60°, 90° and 125° flexion angles; and

FIGS. 4, 6, 8, 10 are planar views of the medial plateau, showing ashatched lines the contact zones of the femoral element with this medialplateau, respectively in the positions of this femoral element and ofthe medial plateau shown on FIGS. 2 and 3, 5, 7 and 9.

FIG. 1 represents a femoral element 1, a medial plateau 2 and a tibialseat 10 of a complete knee prosthesis. The tibial seat 10 shows amedullar keel 11 enabling its anchoring in the tibia, an upper plate 12,intended for receiving the medial plateau 2, and a cylindro-conicalcavity provided in the keel 11.

The femoral element 1 has a curved shape liable to surround the distalend of the femur. It shows rounded external faces and internal facetsintended for resting against the bone, after adequate resection thereof.For its anchoring to the femur, it may include a medullar rod and/orstuds, not represented.

The femoral element 1 forms lateral condyles 3 having, as seen in thesagittal plane, a spiral form corresponding substantially to that of thenatural condyles, i.e. with gradual reduction of their radii towards tothe posterior sectors. Thus, the medial or antero-medial zone 3 a ofthese condyles 3, which is carrying between 0 and approx. 50° of flexion(cf. FIGS. 2 and 3) shows, as seen in the sagittal plane, substantiallycurbed shape of a first radius while the posterior sector 3 b of thecondyles 3 inscribes substantially a circle of a second circle vastlysmaller than said first radius.

In the front plane, these condyles 3 have a convex curved shape.

On the anterior part of the femoral element 1, the condyles 3 delineatebetween themselves an elongated cavity reproducing substantially theanatomic trochlea. At its medial zone, the femoral element 1 includes amedial cage 4, elongated in the antero-posterior direction, whichdelineates a cavity open downwards. This cavity is liable to receive aprominence 5 of the medial plateau 2, described below.

As appears with reference to FIGS. 1 and 2, the femoral element 1 showsa rounded anterior face 1 a which, in hyperextension position of thejoint, rests against the rounded anterior face of the prominence 5.

On its posterior part, the femoral element 1 includes a rounded mediallobe 6. As shown on the Figures, the posterior sector of this lobe 6inscribes a circle of the same centre as the circle inscribed by theposterior sectors 3 b of the condyles 3.

The lobe 6 is arranged at a certain distance from said prominence 5 ofthe plateau 2 in extension position of the prosthesis (cf. FIG. 2) andbeneath a flexion angle of 60° (cf. FIG. 3). From this flexion angle of60° (cf. FIG. 5), this lobe 6 rests against this prominence 5 until theposition of total flexion (cf. FIGS. 7 and 9). In the front plane, thelobe 6 has a curved and convex shape.

The medial plateau 2 is made of a material promoting a slippingmovement, notably of high density polyethylene. It shows a lower face 2a intended for resting against the upper plate 12 of the tibial seat 10and glenoid cavities 2 b receiving the condyles 3 of the femoral element1. These glenoid cavities 2 b have a curvature corresponding to that ofthe zones 3 a of the condyles 3, with which they are consequentlycongruent beneath a flexion angle of 60°. In the front plane, they havea curvature which corresponds to that of the condyles 3 in this sameplane.

The plate 2 includes a cylindro-conical stud 7 protruding in its face 2a, having a diameter slightly smaller than that of the cavity of thekeel 11, which may receive this pivoting stud 7.

As visible on FIGS. 2 and 3, the glenoid surfaces 2 b and the condyles 3are formed in order to be congruent when the prosthesis is in extensionas well as on the portion of the flexion movement extending beneath aflexion angle of 60°. The contact of the condyles 3 with the glenoidsurfaces 2 b extends over zones of substantially same surface area asthat of these glenoid surfaces 2 b, as shown on FIG. 4.

From the flexion angle of 60° and beyond this angle, there appearsgradually a space between the condyles 3 and the surfaces 2 b (cf. FIG.5), resulting from the aforementioned reduction in the radii of thecondyles 3. The contact surface between the condyles 3 and the surfaces2 b is thereby reduced gradually to switch from a maximum extend beneath600 to purely linear contact, as shown on FIG. 6.

The medial prominence 5 forms then a posterior support surface 5 ahaving globally the same curvature as that of the lobe 6, emergingapproximately at the quarter of the width of the plateau 2 from theposterior edge of this plateau and having its apex approximately at onethird of the width of the plateau 2 from the anterior edge of thisplateau. This surface 5 a has a concave shape in the front plane,whereof the curvature corresponds to that of the lobe 6 in this plane.

As appears on FIGS. 5 to 10, at a flexion angle of 60°, the lobe 6contacts the surface 5 a according to a soft and gradual movementresulting from curved shapes complementary of this lobe 6 and of thissurface 5 a. This soft and gradual contact 5 avoids any shock liable tostress the bony anchorings.

From this flexion angle of 60°, the support surface of the lobe 6against the surface 5 a compensates for the loss of congruence of thecondyles 3 with the plateau 2, as shown on FIGS. 6, 8 and 10, whichenables to keep control over the movement of the femoral element 1 withrespect to the tibial seat 10 and conversely; the identical shape of thelobe 6 and of the condyles 3 at the posterior sectors of this lobe 6 andof these condyles 3 enables to perform a “roll-back” movement, i.e. arolling movement of the condyles 3 with respect to the tibial seat inthe posterior direction. This rolling movement enables recall of thelinear contact surfaces of the condyles 3 with respect to the plateau 2,over some ten millimetres in the example represented, as shown on FIGS.6, 8 and 10. This movement enables not only to reproduce the anatomicmovement of the joint, but also to increase the amplitude of the flexionmovement of the joint and not to generate detrimental stresses on thebony anchorings at the end of this flexion movement.

As appears from the foregoing, the invention provides a complete kneeprosthesis with “postero-stabilisation”, exhibiting decisive advantagesof maintaining high congruence between the femoral element and themedial plateau over the whole movement of the joint, while enablingperfectly controlled “roll-back” and without imposing the conservationof the posterior cross ligament.

It goes without saying that the invention is not limited to theembodiment described here above by way of example but it includes,conversely, all the realisation variations covered by the claimsappended below. Notably, the condyles 3 and the plateau 2 may be formedso that the contact of the lobe 6 with the prominence 5 takes place atflexion angles other than 60°, for example 30°, 40° or even 80°; the“roll-back” movement may range, according to the conformation of thelobe 6 and of the prominence 5, from 2 to 20 mm.

1. Complete knee prosthesis, comprising: a femoral element (1) whichfemoral condyles (3) have, seen in the sagittal plane, a spiral formcorresponding substantially to that of the natural condyles, i.e. withgradual reduction of their radii towards posterior sectors, and whichcomprises a medial lobe (6) with a curved surface; a tibial seat formingan upper support plate, and a medial plateau (2) intended for restingagainst said upper support plate of the tibial seat and comprisingglenoid cavities (2 b) receiving the condyles (3) of the femoral element(1), said medial plateau (2) having a medial prominence (5) which formsa posterior support surface (5 a), against which said lobe (6) iscapable of resting during the movement of the joint; prosthesischaracterised in that: the glenoid cavities (2 b) of the medial plateau(2) and the condyles (3) of the femoral element (1) are formed to becongruent when the prosthesis is in extension and on a first part of theflexion movement of the joint extending between this extension positionand the angle of flexion from which the congruence of the condyles (3)with the glenoid surfaces is lost due to the spiral form of the condyles(3); the posterior sectors of the condyles (3) and of the medial lobe(6) inscribe circles of same centres; the medial lobe (6) is formed soas not to contact said posterior support surface (5 a) during said firstpart of movement, but to contact said posterior support surface (5 a)when said flexion angle is reached and then to carry the joint againstthis posterior support surface (5 a) over the remainder of the flexiontravel, while performing a “roll-back”, i.e. a rolling movement of thefemoral condyles (3) with respect to the tibial seat in the posteriordirection, and said posterior support surface (5 a) has a curved shapecorresponding to that of said medial lobe (6), so that this lobe restsagainst this posterior support surface (5 a) over a wide surfacethroughout the second part of movement, consecutive to said first partof movement.
 2. Prosthesis according to claim 1, characterised in thatthe medial plateau (2) is mobile with respect to the tibial seat. 3.Prosthesis according to claim 2, characterised in that the medialplateau (2) is mounted to pivot on the tibial seat by means of a studincluded in the medial plateau (2) or the tibial seat, engaged in acorresponding cavity (7) provided respectively in the tibial seat or themedial plateau (2).